John,

You asked about preamplifier schematics:

We have been using a high gain preamp at our seismic stations for
years in the presence of AM, FM, marine, and Ham transmissions from
the same tower we are borrowing space on for our telemetry. We have
never had a problem with RF interference from these sources. 

The amplifier has a relatively flat response from about 20 hz to 60
seconds, and can be extended to DC for long period instruments.

The schematic is posted at:
http://www.eas.slu.edu/People/STMorrissey/index.html
....stmmisc.html" PSN INFO ... SLU Seismic Network
There are also some photos of the assembled amplifier.

A preamplifier should neither rectify nor amplify an AM signal.
Some have voltage limiting diodes across the input to protect the
amplifier, and some OP-AMPS have these internally, which may be a problem.
But I still doubt that it is an amplifier problem. The fact that the
coil moves when you touch the frame seems to indicate that some
rectification of the noise is occurring elsewhere, possibly in a 
bad ground connection, as I discussed before. Or one side of your
signal from the coil is grounded to a ground loop. I can think of
many possibilities to investigate.

Regards,
Sean-Thomas

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